1. Field of the Invention
This invention relates to optical recording media for memory applications generally and particularly to the use of nitrides of column lll metals as the materials for phase change media for optical recording, especially write once, read many times optical recording.
2. Background
In optical recording for memory applications, a focused incident beam, e.g. from a semiconductor laser, reads or writes bits onto a thin film of appropriate material (media). Writing (encoding) is generally done by either changing the magneto-optic properties of the media or the reflectivity of the media to form a bit, the bit being the region of material irradiated and heated by the focused beam as the beam is scanned (rastered) over the surface of the media and varied in intensity as by being pulsed on and off. In the read operation the laser beam is incident on the bit and the reflected beam is modulated either in terms of a change in polarization (for magneto-optical recording) or reflectivity (for phase change alloys, organic dye/polymer, or ablative media). The present invention addresses phase change media.
Phase change media can be used for applications involving "write once read many" (WORM) as well as erasable optical storage. The present invention is directed primarily to WORM systems. Typical WORM materials currently used are tellurium (Te) or selenium (Se) based films which are based on ablation of material to create reflectivity change. Also used are InSbSn or other metallic alloys that rely on an amorphous to crystalline phase transition for the reflectivity contrast. There are a few drawbacks to these materials. Firstly, since there is the possibility for recrystallization of the phases, stability of storage is an issue. Secondly, Te and Se are unstable in air and are toxic. Thirdly, the reflectivity difference or signal contrast (which is normally defined as .vertline.(R.sub.w -R.sub.s)/(R.sub.w +R.sub.s).vertline. with R.sub.s and R.sub.w being the reflectivity of the starting phase and written phase, respectively) in these materials is in the range of 20-30%. Thus, there is a need for phase change media that offer improved performance relative to the above mentioned three points. Furthermore, it is expected that within the next five years shorter wavelength (600 nm and lower) semiconductor injection lasers will be introduced for higher storage densities, and thus there is need for phase change media compatible with these lasers.